2023 VJC H2 Chem Prelim P3 (Ans)

1 © VJC 2023 9729/03/PRELIM/23 [Turn over CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/03 Paper 3 Free Response Candidates answer on the Question Paper. Additional Materials: Data Booklet 19 September 2023 2 hours READ THESE INSTRUCTIONS FIRST Write your name and CT group on this cover page. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Section A 1 / 24 2 / 16 3 / 20 Section B 4 OR 5 / 20 Total / 80 This document consists of 34 printed pages.

2 © VJC 2023 9729/03/PRELIM/23 Section A Answer all the questions in this section. 1 (a) Describe and explain the trend in thermal stability of the hydrogen halides. [2] Thermal stability decreases down the group . Increase in atomic radius from F to I leading to poorer extent of overlap between the orbital of hydrogen and the orbital of the halogen. Hence H-X bond is weaker, less energy needed to break the bond. (b) A sodium salt of the interhalogen ion, IBrx− is prepared. When heated, the salt evolves bromine vapour, leaving sodium iodide as the residue. The bromine vapour evolved requires 32.0 cm 3 of 0.100 mol dm −3 of aqueous sodium thiosulfate for complete reaction. The balanced ionic equation for the reaction between bromine and thiosulfate is as shown below. S2O32−(aq) + 4Br2(g) + 5H2O(l) → 2SO42−(aq) + 8Br−(aq) + 10H+(aq) (i) Construct a balanced equation for the thermal decomposition of NaIBrx. [1] 2NaIBrx → xBr2 + 2NaI (ii) Calculate the amount of bromine gas evolved. [1] Amount of thiosulfate used = 32.0 ÷ 1000 × 0.100 = 3.20 × 10−3 mol Amount of bromine evolved = 4 × 3.20 × 10−3 = 0.0128 mol The sodium iodide residue was completely reacted with concentrated sulfuric acid to give HI gas, which immediately further reacted with concentrated sulfuric acid to give H2S, together with black crystals of iodine. The mass of the iodine crystals formed was found to be 1.63 g. (iii) Calculate the amount of iodine formed and hence deduce the formula of the interhalogen ion. [2] Amount of iodine formed = 1.63 ÷ (2 × 127) = 6.42 × 10−3 mol Amount of sodium iodide = 6.42 × 10−3 × 2 = 0.0128 mol Amount of bromine atom in the interhalogen ion = 2 × 0.0128 = 0.0256 mol x = 𝒏𝑩𝒓 𝒏𝑰 = 0.0256 ÷ 0.0128 = 2 Thus, the formula of the interhalogen ion is IBr2−. (c) Chloroalkanes and bromoalkanes can be made by the reaction of the corresponding halogen with alkanes. One example is given below. CH3CH2CH3 + Cl2 → CH3CH2CH2Cl + HCl (i) State the conditions needed for this reaction. [1] Excess propane in the presence of UV light (ii) Name and describe the mechanism of this reaction. [2] Free radical substitution Initiation: Cl2 → 2Cl● Propagation:

3 © VJC 2023 9729/03/PRELIM/23 [Turn over Termination: 2Cl● → Cl2 (iii) Suggest why it is not possible to make iodoalkanes by this method. [1] As halogen atom becomes larger , the H-I and C-I bond are weaker . Thus the formation of H-I and C -I bond release insufficient energy to overcome the strong C-H. (d) Suggest a structural formula for each of the compounds , A to D, in the following schemes shown in Figure 1.1. [4] Figure 1.1 (e) Chlorofluoroalkanes, CFCs, were once used as refrigerant fluids and aerosol propellants. In many applications they have now been replaced by alkanes. This is because CFCs contribute to the destruction of the ozone layer. (i) Suggest one reason why CFCs were originally used for these purposes. [1] They liquefied under pressure OR: They volatilised readily when that pressure was released OR: They were inert and non-toxic.

4 © VJC 2023 9729/03/PRELIM/23 (ii) Explain how CFCs destroy the ozone layer. [1] UV radiation homolytically cleaves the C–Cl bond in CFC molecules to generate Cl● radicals which initiate a chain reaction leading to the destruction of ozone molecules. (iii) Suggest one potential hazard of using alkanes instead of CFCs. [1] Alkanes are flammable. (f) Figure 1.2 shows two reactions involving benzene and a suitable Lewis acid as a catalyst. Figure 1.2 (i) Suggest the type of reaction for reaction 1. [1] Electrophilic substitution (ii) Suggest the reagents and conditions for a reaction that could be used to distinguish between Y and Z, and describe the observations. [2] Reagent: Warm or heat with aqueous iodine, and NaOH(aq) Observation: Y gives a pale yellow precipitate of CHI3, while Z does not give any precipitate. (g) When benzene is added to compound Q in the presence of AlCl3, Compound E with the molecular formula of C10H12 is formed. When E is heated with acidified KMnO4, F is formed and effervescence is also observed, F contains 57.8% of carbon, 3.6% of hydrogen and 38.6% of oxygen by mass. The relative molecular mass of F is 166. 1 mol of F readily reacts with 2 mol of NaOH(aq). (i) Determine the molecular formula of F [1] C H O Mass in 100 g 57.8 3.6 38.6 No. of moles 57.8 12 = 4.82 3.6 1.0 = 3.6 38.6 16 = 2.41 Ratio 2 1.49 1 Simplest ratio 4 3 2 ✓ Empirical formula is C4H3O2.

5 © VJC 2023 9729/03/PRELIM/23 [Turn over Molecular formula is (C4H3O2)n. (12 x 4 + 1 x 3 + 16 x 2)n = 166 => n = 2 Molecular formula is C8H6O4 (ii) Hence, suggest the structure for Q, E and F. [3]